Ash is a by-product generated in the combustion of coal. Fly ash is generally captured from the chimneys of power stations and bottom ash is removed from the bottom of the furnace. In the UK, just over 1,000,000 tonnes of fly ash is produced annually.
Worldwide a large proportion of ash produced from coal fired power stations is disposed of in landfill or stored in slag heaps. Some countries impose a tax on the disposal of such waste in landfill. The recycling of ash has become an increasing concern in recent years due to increasing landfill costs as well as environmental costs.
A significant portion of this ash is pozzolanic in nature, which means that when combined with calcium hydroxide it exhibits cementitious properties. In principle fly ash can be used as a replacement for a proportion of Portland cement content of concrete mixtures. Production of Portland cement itself is energy-intensive and produces a large amount of carbon dioxide, approximately one tonne of carbon dioxide per tonne of Portland cement, so replacement of a proportion of this with an otherwise unused by-product could dramatically reduce carbon emissions.
However, ash comprising a high percentage of unburned carbon is not useable as a Portland cement substitute since the ash then has a tendency to adsorb important cementitious chemical admixtures from the concrete during the mixing process. This renders admixtures unavailable to effect their intended purpose. Ash with a carbon content of 7% or less is desirable for use as a pozzolan.
Fly ash can be processed to reduce the carbon content to levels sufficient for use as a pozzolan. Examples of such processes include re-burning the fly ash to reduce the carbon content; electrostatic separation processes which produce low carbon fractions and the chemical treatment of fly ash to minimize the effect of the carbon content by reducing the adsorptive properties of the carbon. All of these processes require at least one additional processing step, adding to the overall cost of producing a useful by-product rather than a waste product.
In Europe there is a legal requirement for power stations to reduce emissions of nitrous and sulphurous oxides, known as NOx and SOx emissions. This has led to coal fired power stations being fitted with low NOx burners. Whilst reducing NOx and SOx emissions, these burners also lead to a slight loss in combustion efficiency which can in turn lead to high levels of carbon in the ash, typically in the region of 20% carbon, rendering the ash an undesirable waste product.
Chinese patent numbers CN1077482, CN1396239, and CN1396239 describe fuel combustion additives. Such additives consist of a range of metals and metal oxides blended in a specific weight proportions. All of these additives are added to the fuel above its standard amount so the amount of fuel used is not reduced.
There also exist millions of tonnes of slag resulting from the extraction of metals from ore.
It would be desirable to provide an improved process for the combustion of coal, providing ash with a low carbon content that renders the ash a desirable and marketable by-product, rather than a waste product that would need to be disposed of in a manner to satisfy environmental regulations. In addition, it would be desirable to provide an improved process in which the amount of coal burned is reduced whilst not reducing the energy output and preferably increasing the energy output and reducing carbon emissions. It would also be desirable to provide a use for slag by-products.